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My Kenwood Chef (1991) does not rotate and unwinds the nut if the food mix is solid (even a pastry mix). I have had it serviced about 10 years ago because this was happening but it still didnt rotate. Its fine with liquids (little resistance)

Janet Bradshaw it is possible that the plastic drive pinion gear is slipping. The plastic version of the gear splits frequently and it will run under no load but once some torque is required, like when it needs to mix stuff, it will slip. We will need to see the gears as well as the gear housing since there are some rivets that create a weak spot and can separate the case. THis guide Disassembling Kenwood Chef A701A Gearbox and Orbital Mixer assembly (could be a different model than yours) shows you an approximation of what could be involved. Adding images to an existing question Update (06/19/2022)

The metal part is actually what screws on the gear shaft for your planetary gearbox. If there would be some sort of mechanical issue in the gearbox and the motor would continue to try and drive the big plastic wheel, the increased resistance would cause the clutch wheels (red arrows) to rotate inside the plastic gear but would not drive the metal bar that connects the clutch wheels to the gear shaft. Looking at the “debris” it doesn’t look like it does that. The debris would not be there. Run the mixer with the cover off and see if the metal moves when no load is applied. After that mix something heavy and see if it then stops the metal part from moving and the two clutch wheels move roll on the arm within the plastic gear. The metal arm should not be moving. Found something. The A902 seems to have the same gearbox as your KM 201

from here

If you had it serviced about 10 years ago, what exactly was serviced? These appliances have electric motors in them, and they used carbon brushes.Those brushes are considered wear-parts, and needs to be replaced in intervals, depending on frequency of use.Along with replacing the brushes, it’s good practice to clean all contact points as the wearing of the brushes collect and needs to be cleaned to provide good contact. One important symptom of bad / worn brushes, is the loss of power in the appliance, which is what you describe.